1. Field of the Invention
The present invention relates to an encoding device and method for converting an m-bit data word into an n-bit code word. In addition, the invention relates to a recording device and method for recording a recording code string obtained by encoding and a recording medium. In addition, the invention relates to a decoding device and method for decoding a recording code string.
2. Description of the Related Art
As optical recording media for performing recording/reproduction of a signal by light irradiation, for example, so-called optical discs such as a Compact Disc (CD), Digital Versatile Disc (DVD) or Blu-ray Disc (BD) (registered trademark) have come into wide use.
In the optical discs widely used in the present state, mark edge recording for defining a recording code as Non Return to Zero Inverting (NRZI) and performing recording after conversion into Non Return to Zero (NRZ) code upon recording is performed.
In the optical discs, due to the relationship for obtaining a tracking error signal from a groove, pit, or the like, few low band components of a recording code are necessary. That is, a tracking servo band is lower than a signal band of a recording code. However, if there are many low band components in the recording code, the components of the recording code may be superimposed on the tracking error signal such that the tracking servo characteristics are deteriorated.
Accordingly, in the optical disc of the related art, the low band components of the recording code are suppressed by controlling the absolute value of the Digital Sum Value (DSV) of the recorded NRZ code string to be decreased.
For example, in a CD, an encoding method of using an EFM modulation code, satisfying a limit of a minimum run length d=2 between a 14-bit code word and the next code word, and selecting and inserting predetermined 3 connection bits so as to decrease the absolute value of the DSV of the code string is used.
In a DVD, using a modulation code called EFM Plus, with respect to a certain data word, a code word of a main table and a substitution table which decreases the absolute value of the DSV of the code string is selected and encoded so as to perform DSV control. This modulation code is described, for example, in Kees A. Schouhamer Immink “EFMPlus: THE CODING FORMAT OF THE MULTIMEDIA COMPACT DISC”, IEEE Transaction on Consumer Electronics, Vol. 41, Issue 3, August 1995 and International Publication No. 95/22802.
In a BD, a modulation code called 17PP is used, but a DC control bit is periodically defined in a recording data format of the BD. Thus, encoding is performed after the DC control bit of “0” or “1”, which decreases the absolute value of the DSV of the code string, is selected.
With respect to an optical disc which is widely used in the present state of the CD, the DVD, the BD and the like, first, as a next-generation optical disc, the present applicant proposes a so-called bulk recording type optical disc (simply referred to as a bulk type) described in Japanese Unexamined Patent Application Publication No. 2008-135144 or Japanese Unexamined Patent Application Publication No. 2008-176902.
Here, bulk recording indicates, for example, a technology of realizing a large amount of recording capacity by irradiating a laser beam to an optical recording medium (bulk type recording medium 100) having at least a cover layer 101 and a bulk layer (recording layer) 102 while sequentially changing a focal point position so as to perform multi-layer recording in the bulk layer 102, as shown in FIG. 26.
In such bulk recording, Japanese Unexamined Patent Application Publication No. 2008-135144 discloses recording technology which is a so-called micro hologram method. In the micro hologram method, a so-called hologram recording material is used as a recording material of the bulk layer 102. As the hologram recording material, for example, a photopolymerizable photopolymer or the like is widely used.
The micro hologram method is classified broadly into a positive type micro hologram method and a negative type micro hologram method.
The positive type micro hologram method is a method of focusing two opposing light fluxes (light flux A and light flux B) at the same position so as to form a minute interference fringe (hologram) and using the minute interference fringe as a recording mark.
The negative type micro hologram method is a method of erasing an interference fringe formed in advance by laser beam irradiation and using the erased portion as a recording mark, in opposition to the positive type micro hologram method. In the negative type micro hologram method, a process for forming an interference fringe in the bulk layer is performed in advance, as an initialization process.
The present applicant proposes, for example, a recording method of forming a void (blank or hole) disclosed in Japanese Unexamined Patent Application Publication No. 2008-176902 as a recording mark, as a bulk recording method different from the micro hologram method.
The void recording method is, for example, a method of irradiating a laser beam to the bulk layer 102 formed of a recording material such as a photopolymerizable photopolymer with relatively high power so as to record a blank in the bulk layer 102. As described in Japanese Unexamined Patent Application Publication No. 2008-176902, the formed blank portion has a refractive index different from that of the other portion of the bulk layer 102 and thus the light reflection ratio of the boundary portion thereof is increased. Accordingly, the blank portion functions as a recording mark and thus information recording by formation of a blank mark is realized.
In such a void recording method, since the hologram is not formed, recording is completed by light irradiation from one side. That is, as in the positive type micro hologram method, it is not necessary to focus two light fluxes at the same position so as to form the recording mark.
In addition, in comparison with the negative type micro hologram method, it is an advantage that the initialization process is not performed.
In Japanese Unexamined Patent Application Publication No. 2008-176902, although an example of irradiating a pre-cure light before recording at the time of performing void recording is described, void recording is possible even when the irradiation of the pre-cure light is omitted.
However, the bulk recording type (also simply referred to as bulk type) optical disc recording medium is proposed as the above various recording methods, but the recording layer (bulk layer) of the bulk type optical disc recording medium does not have an explicit multi-layer structure in the sense that, for example, a plurality of reflection films is formed. That is, in the bulk layer 102, a reflection film and a guide groove are not provided in every recording layer included in a general multi-layer disc.
Accordingly, in the structure of the bulk type recording medium 100 as it is shown in FIG. 26, focus servo and tracking servo may not be performed during recording in which the mark is not formed.
Accordingly, practically, in the bulk type recording medium 100, a reflection surface (reference surface) is provided which becomes a reference having guide grooves shown in FIG. 27.
More specifically, the guide grooves (position guide elements) such as pits or grooves are formed in a lower surface side of the cover layer 101 in a spiral shape or a concentric shape and a selective reflection film 103 is formed on the guide grooves. The bulk layer 102 is laminated on the lower layer side of the cover layer 101, on which the selective reflection film 103 is formed, with an adhesive material interposed therebetween as an intermediate layer 104 in the figure, such as a UV curing resin.
Here, through the formation of the guide grooves such as pits or grooves, for example, recording of absolute position information (address information) such as radius position information or rotation angle information is performed. In the following description, a surface (in this case, a surface on which the selective reflection film 103 is formed) in which such guide grooves are formed and the absolute position information is recorded is referred to as a “reference surface Ref”.
After such a medium structure is formed, as shown in the figure, not only a laser beam (hereinafter, referred to as a recording/reproduction laser beam or simply a recording/reproduction light) for recording (or reproducing) a mark but also a servo laser beam (simply referred to as a servo light) as a laser beam for position control is irradiated to the bulk type recording medium 100 through a common objective lens.
At this time, if the servo laser beam reaches the bulk layer 102, the mark recording in the bulk layer 102 may be adversely affected. Accordingly, in the bulk recording method of the related art, the laser beam having a wavelength range different from that of the recording/reproduction laser beam is used as the servo laser beam, and the selective reflection film 103 having wavelength selectivity, which reflects the servo laser beam and transmits the recording/reproduction laser beam, is provided as the reflection film formed on the reference surface Ref.
On the above assumption, the operation at the time of mark recording for the bulk type recording medium 100 will be described. First, when multi-layer recording is performed with respect to the bulk layer 102 in which the guide grooves and the reflection film are not formed, the layer position where the mark is recorded in a depth direction in the bulk layer 102 is set in advance. In the figure, the case where a total of 5 information recording layer positions L including a first information recording layer position L1 to a fifth information recording layer position L5 are set as a layer position (mark forming layer position; also referred to as an information recording layer position) where the mark is formed in the bulk layer 102 is shown. As shown, in the information recording layer position L, the first information recording layer position L1 is provided at an uppermost side and, next, the information recording positions L2, L3, L4 and L5 are sequentially provided toward a lower layer side.
During recording in which the mark is not yet formed, a focus servo control and a tracking servo control are not performed based on the reflected light of the recording/reproduction laser beam with respect to the layer positions in the bulk layer 102 as a target. Accordingly, the focus servo control and the tracking servo control of the objective lens during recording are performed so as to enable the spot position of the servo laser beam to follow the guide grooves on the reference surface Ref based on the reflected light of the servo laser beam.
It is necessary for the recording/reproduction laser beam to reach the bulk layer 102 formed on the lower layer side of the selective reflection film 103 than the reference surface Ref and to select the focusing position in the bulk layer 102, for mark recording. To this end, in an optical system in this case, a focus mechanism (expander) for the recording/reproduction light is provided, which independently adjusts a focusing position of the recording/reproduction laser beam, separately from the focus mechanism of the objective lens.
That is, the focusing position of the recording/reproduction laser beam is adjusted independently of the servo laser beam, by changing collimation of the recording/reproduction laser beam incident to the objective lens by the provided expander.
The position of the recording/reproduction laser beam in the tracking direction is automatically controlled to a position just below the guide grooves in the reference surface Ref by the tracking servo of the objective lens using the above servo laser beam.
In addition, when reproduction is performed with respect to the bulk type recording medium 100 in which mark recording is already performed, it is not necessary to control the position of the objective lens based on the reflected light of the servo laser beam, as during recording. That is, during reproduction, focus servo control and tracking servo control of the objective lens based on the reflected light of the recording/reproduction laser beam are performed, using a mark row formed at the information recording layer position L (also referred to as an information recording layer L or a mark formation layer L, during reproduction) to be reproduced as a target.